1. Field of the Invention
The present invention relates to non-destructive testing of ropes, cables, and metal strands for flaws and tension.
2. Description of the Prior Art
Non-destructive evaluation (NDE) of ropes is known in the art. Some NDE methods are in practice, while other methods have been proposed, but are not yet perfected. As will be shown hereinafter, no NDE method combines the advantageous features of the transverse impulse vibrational wave method disclosed in this application.
In an article by James H. Williams, Jr., John Hainsworth, and Samson S. Lee entitled "Acoustic-Ultrasonic Nondestructive Evaluation of Double Braided Nylon Ropes Using the Stress Wave Factor" which appeared in Fibre Science and Technology, 21 (1984), pp. 169-180, experimentation is performed on synthetic ropes with the object of constructing an analytical model wherein ultrasonic wave conductivity (Stress Wave Factor) of a rope is a function of the condition of the rope and the tension on the rope. It is proposed that such a model would enable accurate testing of ropes for flaws by measuring Stress Wave Factors. To data, no Stress Wave Factor model has been proposed having reliable utility for rope testing. The variation in the relationship between Stress Wave Factor, tension, and rope condition among different rope compositions and structures is not yet fully understood.
Even if an adequate model for interpreting Stress Wave Factor test results were found, the utility of Stress Wave Factor testing would not compare favorably with the transverse impulse vibrational method. While the transverse impulse vibrational wave method permits testing the entire length of a rope from a single test site near one of its ends, Stress Wave Factor method tests only a short length of a synthetic rope because synthetic ropes quickly dissipate the energy of the vibrations used in Stress Wave Factor testing.
Electromagnetic NDE are presently the only type of non-visual method which is in current, widespread practice. Electromagnetic NDE methods are discussed in an article by Herbert R. Weischedel entitled "The Inspection of Wire Ropes in Service: A Critical Review" appearing in Materials Evaluation, 43, December 1986, pp 1592-1605.
Electromagnetic NDE methods are used for: (1) localized fault detection (L.F.) and (2) loss of metallic cross-sectional area testing (L.M.A.)
Electromagnetic NDE methods are limited to use on ferromagnetic materials, unlike transverse impulse vibrational wave method which may be performed on ferromagnetic or non-ferromagnetic materials as well as synthetic materials.
L.F. testing is based on the principal that broken wires in a wire rope made of ferromagnetic steels distort a magnetic flux passing the point of breakage causing magnetic flux leakage which is detectable in the area surrounding the rope. L.F. testing is conducted by positioning a strong permanent or electromagnet in close proximity to a wire rope to be tested. As the rope passes the magnet, or the magnet is moved along the length of the rope, a magnetic flux is initiated in the length of rope adjacent to the pole interspace of the magnet. Differential sensing coils are positioned around the rope to detect magnetic flux leakage.
Only major flaws, such as broken wires and severe corrosion pitting, are detected by L.F. testing, because only substantial changes in the magnetic flux leakage are detected by the differential sensors. Small flaws, or widely dispersed flaws, do not produce substantial and rapid magnetic flux leakage changes and are often missed using L.F. testing.
L.M.A. testing involves direct measurement of magnetic flux through a length of a wire rope. Variation in the magnetic flux through different portions of a single rope indicate a change in the cross-sectional area of the rope, which, in turn, indicates possible deterioration of the rope at areas of decreased cross-sectional area.
The electromagnetic methods require passing the entire length of a metallic rope to be tested through the testing apparatus or the testing apparatus be moved along the entire length of the rope. As with Stress Wave Factor testing, the necessity for access to the entire length of a rope reduces the utility of electromagnetic NDE methods.
Methods based on measuring vibrational frequencies of ropes and cables for determining tension are also known in the art. U.S. Pat. No. 4,565,099 issued to Arnold, U.S. Pat. No. 4,376,368 issued to Wilson, and U.S. Pat. No. 4,158,962 issued to Conoval each related to calculating the tension on a rope or cable as a function of its fundamental frequency of vibration. The equipment and methods shown in these patents and otherwise known in the art are not, however, suitable for practicing the non-tension related aspects of the transverse impulse vibrational wave method as described herein.
It would, therefore, be advantageous to develop an NDE method having utility for testing ferromagnetic and non-ferromagnetic ropes alike, which would require access to only a limited portion of the rope to be tested, which would detect minor as well as major rope flaws, and which would permit calculating tension on ropes without additional equipment.